PSUBSCRIBE for pattern matching in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What messages does this PSUBSCRIBE receive?

Given the Redis command PSUBSCRIBE news.*, which published messages will the subscriber receive?

Redis

PSUBSCRIBE news.*

PUBLISH news.sports "Sports update"
PUBLISH news.weather "Weather update"
PUBLISH news "General news"
PUBLISH news.sports.football "Football news"

A Receives messages from channels: news only

B Receives messages from channels: news, news.sports, news.weather

C Receives messages from channels: news, news.sports.football

D Receives messages from channels: news.sports, news.weather, news.sports.football

💡 Hint

Remember that the asterisk (*) matches any characters in the channel name after the prefix.

The pattern 'news.*' matches any channel starting with 'news.' followed by any characters. So it matches 'news.sports', 'news.weather', and 'news.sports.football'. It does not match 'news' alone because there is no dot after 'news'.

📝 Syntax

intermediate

Identify the correct PSUBSCRIBE command syntax

Which of the following Redis commands correctly subscribes to all channels starting with 'chat.'?

A PSUBSCRIBE chat.*

B SUBSCRIBE chat.*

C PSUBSCRIBE chat

D SUBSCRIBE chat

💡 Hint

PSUBSCRIBE supports patterns, SUBSCRIBE does not.

PSUBSCRIBE is used for pattern matching subscriptions. The pattern 'chat.*' matches all channels starting with 'chat.'. SUBSCRIBE does not support patterns and treats the argument as a literal channel name.

❓ optimization

advanced

Optimizing pattern subscriptions for multiple similar channels

You want to subscribe to all channels related to user notifications, which are named like 'user.123', 'user.456', 'user.789'. Which PSUBSCRIBE pattern is the most efficient and correct?

A PSUBSCRIBE user.?

B PSUBSCRIBE user.*

C PSUBSCRIBE user.[0-9]*

D PSUBSCRIBE user

💡 Hint

Consider how Redis pattern matching works with wildcards.

The pattern 'user.*' matches any channel starting with 'user.' followed by any characters, which covers all user notification channels. 'user.?' matches only channels with exactly one character after 'user.'. 'user.[0-9]*' requires the character immediately after 'user.' to be a digit followed by any characters, which is overly restrictive. 'user' matches only the exact channel 'user'.

🔧 Debug

advanced

Why does this PSUBSCRIBE pattern not receive expected messages?

A developer runs PSUBSCRIBE user.*.notifications but does not receive messages published to 'user.123notifications'. What is the likely cause?

A The PSUBSCRIBE command requires escaping dots, so the pattern should be 'user\. *.notifications'.

B Redis patterns treat '*' as matching only one character, so it does not match '123'.

C The pattern 'user.*.notifications' does not match 'user.123notifications' because Redis patterns do not support multiple wildcards separated by dots.

D The pattern is correct; the issue is that messages are not published to 'user.123notifications'.

💡 Hint

Check how Redis pattern matching treats the '*' wildcard.

In Redis, the '*' wildcard matches any sequence of characters including dots. However, the pattern 'user.*.notifications' expects a literal dot after the '*', so it matches channels starting with 'user.', followed by any characters (including dots), then a dot, then 'notifications'. But since '*' can match dots, the pattern effectively expects two dots, which does not match 'user.123notifications' as expected. The correct pattern to match 'user.123notifications' is 'user.*notifications' without the extra dot.

🧠 Conceptual

expert

Understanding PSUBSCRIBE pattern matching behavior

Which statement correctly describes how Redis PSUBSCRIBE pattern matching works?

A The '*' wildcard matches any sequence of characters, including dots, and patterns are matched against the entire channel name.

B The '*' wildcard matches only characters up to the first dot, so patterns cannot match across dots.

C Patterns in PSUBSCRIBE are matched only against the prefix of channel names, not the entire name.

D PSUBSCRIBE patterns require regular expressions syntax for matching channel names.

💡 Hint

Think about how the '*' wildcard behaves in Redis channel patterns.

In Redis PSUBSCRIBE, the '*' wildcard matches any sequence of characters, including dots. Patterns are matched against the entire channel name, not just a prefix. Redis does not use regular expressions for PSUBSCRIBE patterns.

Practice

1. What does the Redis command PSUBSCRIBE do?

easy

A. It subscribes to channels matching a pattern with wildcards.

B. It unsubscribes from all channels.

C. It publishes messages to a channel.

D. It deletes keys matching a pattern.

Solution

1. Step 1: Understand the purpose of PSUBSCRIBE

   PSUBSCRIBE listens to messages from channels that match a given pattern, using wildcards like '*'.

2. Step 2: Compare with other commands

   Unlike PUBLISH or UNSUBSCRIBE, PSUBSCRIBE focuses on pattern-based subscription.

3. Final Answer:

   It subscribes to channels matching a pattern with wildcards. -> Option A

4. Quick Check:

   PSUBSCRIBE = pattern subscription [OK]

Hint: PSUBSCRIBE listens to patterns, not single channels [OK]

Common Mistakes:

• Confusing PSUBSCRIBE with PUBLISH
• Thinking PSUBSCRIBE deletes keys
• Mixing PSUBSCRIBE with UNSUBSCRIBE

2. Which of the following is the correct syntax to subscribe to all channels starting with 'news.' using PSUBSCRIBE?

easy

A. PSUBSCRIBE news?

B. PSUBSCRIBE news.*

C. PSUBSCRIBE news#

D. PSUBSCRIBE news+

Solution

1. Step 1: Identify the wildcard for pattern matching

   In Redis PSUBSCRIBE, '*' matches any sequence of characters.

2. Step 2: Match the pattern for channels starting with 'news.'

   The pattern 'news.*' matches all channels beginning with 'news.' followed by anything.

3. Final Answer:

   PSUBSCRIBE news.* -> Option B

4. Quick Check:

   '*' wildcard = correct syntax [OK]

Hint: Use '*' to match any characters in PSUBSCRIBE patterns [OK]

Common Mistakes:

• Using '?' which matches exactly one character
• Using '#' or '+' which are not Redis wildcards
• Missing the dot after 'news'

3. Given the following Redis commands executed in order:

PSUBSCRIBE sports.*
PUBLISH sports.football "Goal!"
PUBLISH sports.basketball "Score!"
PUBLISH news.weather "Rain"

Which messages will the subscriber receive?

medium

A. "Goal!" and "Score!"

B. "Goal!" only

C. "Rain" only

D. All three messages

Solution

1. Step 1: Understand the subscription pattern

   The subscriber listens to channels matching 'sports.*', meaning any channel starting with 'sports.'.

2. Step 2: Check published messages against the pattern

   Messages on 'sports.football' and 'sports.basketball' match the pattern; 'news.weather' does not.

3. Final Answer:

   "Goal!" and "Score!" -> Option A

4. Quick Check:

   Pattern matches sports.* channels only [OK]

Hint: Only messages on matching channels are received [OK]

Common Mistakes:

• Assuming all published messages are received
• Confusing exact channel with pattern
• Ignoring the pattern prefix

4. You run the command PSUBSCRIBE sports.* but receive no messages when publishing to sports.football. What is the most likely mistake?

medium

A. You published messages before subscribing.

B. You used SUBSCRIBE instead of PSUBSCRIBE.

C. You subscribed to sports.* but published to sportsfootball without a dot.

D. You used the wrong Redis server version.

Solution

1. Step 1: Check the channel names for exact pattern match

   The pattern 'sports.*' matches channels starting with 'sports.' including the dot.

2. Step 2: Identify mismatch in published channel

   Publishing to 'sportsfootball' (no dot) does not match 'sports.*' pattern.

3. Final Answer:

   You subscribed to sports.* but published to sportsfootball without a dot. -> Option C

4. Quick Check:

   Pattern requires exact prefix including dot [OK]

Hint: Check exact channel names match pattern including dots [OK]

Common Mistakes:

• Confusing SUBSCRIBE with PSUBSCRIBE
• Ignoring dot in channel names
• Assuming order of commands affects delivery

5. You want to listen to all channels that contain the word 'chat' anywhere in their name using PSUBSCRIBE. Which pattern should you use?

hard

A. chat

B. chat*

C. *chat

D. *chat*

Solution

1. Step 1: Understand the wildcard '*' usage

   '*' matches any sequence of characters, including none.

2. Step 2: Create pattern to match 'chat' anywhere

   Using '*chat*' matches any channel with 'chat' anywhere in its name.

3. Final Answer:

   *chat* -> Option D

4. Quick Check:

   Use '*' before and after 'chat' to match anywhere [OK]

Hint: Use '*' before and after word to match anywhere [OK]

Common Mistakes:

• Using 'chat' alone matches exact channel only
• Using 'chat*' matches only channels starting with 'chat'
• Using '*chat' matches only channels ending with 'chat'